This application claims the benefit of priority from the prior Japanese Patent Application No. 2000-400814, filed on Dec. 28, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention generally relates to an inspection system and a method for controlling the same. More specifically, the invention relates to a substrate inspection system for observing an integrated circuit pattern on a semiconductor wafer by using charged particle beams.
2. Description of the Related Art
With the high integration of LSIs, the detection sensitivity required to detect defects and foreign matters on wafers and masks are rising. It is said that the detection sensitivity must be generally xc2xd or less of a pattern wiring width in order to inspect pattern defects and foreign matters which cause critical failures. For that reason, in a defect inspection on a semiconductor wafer which has a design rule of 0.13 xcexcm or less, the limitation of the pattern defect inspection based on optical systems is realized. Under such a background, pattern defect inspection systems using charged particle beams have been developed (Japanese Patent Laid-Open Nos. 5-258703, 6-188294, 7-249393, etc.). In order to achieve a high-speed processing in a semiconductor wafer pattern inspection system based on charged particle beams, it is expected that the construction of an electron optical system proposed by Japanese Patent Laid-Open No. 7-249393 is the most influential means. In order to realize it, there is an optical system proposed by Japanese Patent Laid-Open No. 11-132975.
Referring to FIG. 3, a technique described in Japanese Patent Laid-Open No. 11-132975 will be described as an example of a related art. Furthermore, the same reference numbers are given to the same portions in the respective figures, and the detailed descriptions thereof are omitted.
A substrate inspection system shown in FIG. 3 uses electron beams as charged particle beams, and schematically comprises: an electron beam irradiation part, and a control part thereof; a stage 12 for mounting thereon a substrate 11 serving as a sample, and a control part thereof; a secondary, reflected and back-scattered electron beam mapping projecting optical part (which will be hereinafter simply referred to as a mapping projecting optical part), and a control part thereof; an electron image detecting part, and a control part thereof; and an electron beam deflecting part, and a control part thereof.
The electron beam irradiation part is arranged so as to be mechanically inclined with respect to the surface of the substrate 11. On the other hand, the optical axis of the mapping projecting optical part is arranged so as to extend in a direction perpendicular to the surface of the substrate 11. With this construction, electron beams (which will be hereinafter referred to as irradiation electron beams) 31 emitted from an electron gun enter the electron beam deflecting part in a direction inclined by a predetermined angle with respect to the surface of the sample, and the irradiation electron beams 31 are deflected by the electron beam deflecting part in a direction perpendicular to the surface of the substrate 11 to enter the substrate 11. In addition, secondary electrons, reflected electrons and back-scattered electrons (which will be hereinafter referred to as secondary electrons and so forth), which are produced on the surface of the substrate 11, are accelerated by the electric field on the surface of the substrate 11 in a direction perpendicular to the surface of the substrate 11 and enter the mapping projection optics.
The electron beam irradiation part comprises an electron gun and two-stage quadrupole lenses. The electron gun includes a lanthanum hexaboride (which will be hereinafter referred to as LaB6) cathode 1 having a rectangular electron emission surface with the size of 100 xcexcmxc3x9710 xcexcm, an Wehnelt electrode 2 having a rectangular opening, an anode 3 having a rectangular opening, and a deflecting system 4 for adjusting an optical axis. The accelerating voltage, emission current and optical axis for the irradiation electron beams 31 are controlled by control parts 7, 8 and 9, respectively. In order to form rectangular beams having a size of 100 xcexcmxc3x9725 xcexcm on the surface of the substrate 11, two-stage electrostatic quadrupole lenses 5 and 6, and a control part 10 thereof are provided. The accelerating voltage for the irradiation electron beam 31 is determined by the relationship between the resolution of the mapping projecting optical part and the incidence voltage to the substrate 11.
The irradiation electron beams 31 are emitted from the LaB6 cathode 1 to leave the electron beam irradiation part while being converged by the quadrupole lenses 5 and 6, and enter the electron beam deflecting part 34. The electron beam deflecting part 34 has a Wien filter (not shown). The trajectory of irradiation electron beams 31 is deflected by the Wien filter so as to be perpendicular to the surface of the substrate 11, and then, the irradiation electron beams 31 leave the electron beam deflecting part 34. Thereafter, the irradiation electron beams 31 are reduced by a rotationally symmetric electrostatic lens 14 to perpendicularly irradiate the substrate 11. A voltage is applied to the electrostatic lens 14 by a power supply 15.
A negative voltage is applied to the stage 12 by a power supply 13, so that a negative voltage is applied to the substrate 11. The movement of the stage 12 is controlled by a control part 13. The value of the voltage applied to the substrate 11 is determined by the resolution performance of the mapping projecting optical part. In order to obtain a resolution of 0.1 xcexcm or less, the voltage of electron beams of secondary ions (which will be hereinafter referred to as secondary electron beams) 32 require energy of about 5 keV, so that a voltage applied to the sample is preferably 5 kV. On the other hand, the energy of the irradiation electron beams 31 is determined by the difference between the voltage applied to the substrate and the incident voltage to the substrate. If the substrate 11 is a semiconductor wafer, the incident voltage to the substrate 11 is generally about 800 V or less to prevent the irradiation damage and charging. As a result, the voltage of the irradiation electron beams is 5.8 kV.
When the wafer is irradiated with the irradiation electron beams 31, secondary electrons and so forth forming an electron image indicative of the shape, material, potential and so forth of the surface of the substrate are emitted from the surface of the substrate 11. These electrons are accelerated by an accelerating field produced between the electrostatic lenses 14 and enter the electron beam deflecting part 34 while drawing a trajectory having a focal point at infinity by the electrostatic lens 14. The electron beam deflecting part 34 is controlled on the conditions that the secondary electron beams 32 incident from the substrate 11 are caused to travel straight, and the secondary electron beams travel straight in the deflecting part 34 to enter a spectral means. Of energy of the secondary electron beams 32 produced from the substrate 11, only secondary electron beams having energy of a predetermined value or more enter the mapping projecting optical part.
The mapping projecting optical part includes three-stage rotationally symmetric electrostatic lenses 16, 18 and 20. The secondary electron beams 32 are enlarged to be projected by the electrostatic lenses 16, 18 and 20 to form an image on the electron image detecting part. The control parts 17, 19 and 21 control the voltages of the electrostatic lenses 16, 18 and 20, respectively.
The electron image detecting part includes an MCP (Micro Channel Plate) detecting device 22, a fluorescent screen 23, an FOP (Fiber Optical Plate) 24, a CCD element 106 and a CCD camera 25. In this embodiment, the CCD element 106 is a CCD area sensor. The secondary electron beams 32 entering the MCP detecting device 22 are multiplied 10,000 to 100,000 times by the MCP to irradiate the fluorescent screen 23, so that a fluorescent image is produced on the fluorescent surface. Thus, the electron image on the surface of the substrate 11 by means of the electron beams is converted into an optical image. This fluorescent image is detected by the CCD area sensor via the FOP 24 to be outputted as a picture signal from the CCD camera 25. The picture signal is fed to a host computer 29 via a signal control part 28, so that the image processing and the storage of image data are carried out. In addition, image data are displayed on a display 30 in the form of a two-dimensional image.
However, in the conventional substrate inspection system, there is a problem in that the image deteriorates in a process for incorporating the electron image. Referring to FIGS. 4 and 5, this point will be described below. FIG. 4 is an illustration for explaining a technique for incorporating an image in the above described substrate inspection system described in Japanese Patent Laid-Open No. 11-132975. The electron image formed by the secondary electron beams 32 forms an image on the incident surface of the MCP 22 by electrostatic lenses 20a, 20b and 20c which are positioned at the final stage of the mapping projecting optical part. In more detail, the electron image detecting part includes MCPs 22a, 22b, the fluorescent screen 23, the FOP 24, the CCD element 106 and the CCD camera 25. In order to install the MCP 22 and the fluorescent screen 23 in a vacuum electron beam column, the separation of vacuum from the atmosphere is herein realized by clamping the FOP 24 by vacuum vessel walls 112. In order to obtain a higher gain, the MCP 22 comprises the combination of the two-stage MCPs 22a and 22b. The MCP 22 comprises a bundle of glass tubes which adhere to each other and each of which has an inside diameter of 10 xcexcm or less and an air-core extending in longitudinal directions. On the inner surface of the tube, a material having high secondary electron emission efficiency is applied. The electron incident surface (the side of 22a) of the MCP 22 is grounded. On the other hand, the outgoing surface (the side of the MCP 22b) thereof is held at a positive potential by a power supply 54. Thus, an accelerating field extending from the incident surface to the outgoing surface exists in the tube. Electrons incident on the MCP 22 are multiplied while repeating scattering in each of the tubes of the first-stage MCP 22a, leave the outgoing surface of the MCP 22a and enter the second-stage MCP 22b to be further multiplied. Electrons emitted from the outgoing surface of the second-stage MCP 22b are accelerated toward the fluorescent surface of the fluorescent screen 23 by an accelerating field which is formed by a power supply 52, to irradiate the fluorescent surface to cause the fluorescent surface to emit light. Thus, the electron image formed on the incident surface of the MCP 22 can be converted into an optical image by causing light to be emitted from the fluorescent surface of the fluorescent screen 23 while multiplying the electron image by the MCP 22. The MCP 22 is a very effective means for improving S/N of the electron image since the MCP 22 can multiply electrons while maintaining the spatial resolution. The optical image produced on the fluorescent surface of the fluorescent screen 23 is detected by the CCD element 106 via the FOP 24 to be outputted as a picture signal from the CCD camera 25.
If a relay lens of an optical lens is adopted between the CCD element 106 and the fluorescent surface in order to incorporate the optical image, which is produced on the fluorescent surface, into the CCD element 106 in such an electron image detecting system, the optical system tends to be made large in order to prevent distortion. This is an obstacle to the request for the miniaturization of the system. In addition, the decrease of the transmittance due to the optical lens itself can not be ignored. The above described substrate inspection system is characterized in that the FOP 24 is adopted in this region to shorten the distance between the fluorescent surface of the fluorescent screen 23 and the CCD element 106 and to improve the transmittance. However, it is not easy to suppress distortion due to the method for producing the FOP. As shown in FIG. 5, distortions, such as a share distortion D1 and a gross distortion D2, exist in the FOP 24. It is very difficult to reduce these distortions. It is known that such distortions depend on the distance between the incident surface and outgoing surface of the FOP 24 and decrease in proportion to the decrease of the distance. However, the FOP 24 also has the function of causing the CCD element 106 to be electrically insulated from the light receiving surface by applying a high voltage of about +4 kV to the fluorescent surface. From this point, it is difficult to produce the FOP 24 so that the FOP 24 has a thickness of 5 mm or less. In the current FOP producing method, it is difficult to suppress the distortion of a FOP having a thickness of 5 mm to be 10 xcexcm or less. This mainly causes image distortion. This means that if the pixel size of the CCD element is 16 xcexcm, the distortion of the FOP is 0.625 pixels, and the resolution of the image of a region, in which the FOP is distorted, greatly deteriorates. Due to this partial deterioration of resolution, a false defect may occur in a defect inspection. In addition, with respect to both of the optical lens and the FOP, any optical component for transferring the optical image is provided between the fluorescent surface and the CCD element 106, so that it is not possible to avoid the deterioration of the spatial resolution. In the optical system shown in FIG. 4, the image is transferred from the MCP 22, FOP 24 and CCD camera 25, so that it is not possible to avoid the deterioration of the spatial resolution of the image. If an image having a line and space pattern of 0.2 xcexcm is acquired by pixels of 1 xcexcm, the MTF (Modulation Transfer Function) values, which are indexes indicative of optical characteristics, of the MCP 22, the FOP 24 and the CCD camera 25 are 0.43, 0.8 and 0.5, respectively. The MTF from the MCP 22 to the CCD camera 25 is a product of the respective MTFs (MTFdetector=MTFMCPxc3x97MTFFOPxc3x97MTFCCD), so that MTFdetector=0.172 which is greatly deteriorated. In order to suppress such distortion of the MTF, it is required to simplify the construction for incorporating an optical image after an electron image is formed on the MCP 22.
According to a first aspect of the invention, there is provided a substrate inspection system comprising: a substrate mounting part for mounting thereon a substrate to be inspected; a charged particle beam irradiation part for generating a charged particle beam to irradiate the substrate with the charged particle beam, the irradiation of the charged particle beam causing a secondary charged particle and/or a reflected charged particle to generate from the substrate; an electron image detecting part for detecting an electron image which is formed by the secondary charged particle and/or the reflected charged particle and is indicative of a physical property of the surface part of the substrate and for outputting a picture signal of the image; the electron image detecting part including a charged particle multiplying device for multiplying the secondary charged particle and/or the reflected charged particle, and an image grabbing element having a fluorescent body for receiving the multiplied secondary charged particle and/or reflected charged particle as the electron image and for converting the electron image into an optical image, the image grabbing element converting the optical image into the picture signal; the charged particle multiplying device having an entrance surface through which the secondary charged particle and/or the reflected charged particle enter; the fluorescent body having a light receiving surface for receiving the multiplied secondary charged particle and/or reflected charged particle and a fluorescent surface on which the optical image appears; a mapping projecting part for projecting the secondary charged particle and/or the reflected charge particle in some degree of magnification on the electron image detecting part; an inspection part for inspecting the substrate on the basis of the picture signal; and a control part for causing the fluorescent surface of the fluorescent body to be grounded and for applying a first negative potential to the entrance surface of the charged particle multiplying device.
According to a second aspect of the invention, there is provided a method for controlling a substrate inspection system comprising: a substrate mounting part for mounting thereon a substrate to be inspected; a charged particle beam irradiation part for generating a charged particle beam to irradiate the substrate with the charged particle beam; an electron image detecting part for detecting an electron image which are formed by a secondary charged particle and/or a reflected charged particle which are produced from the substrate by irradiation with the charged particle beam and which is indicative of a physical property of the surface part of the substrate and for outputting a picture signal of the image; a mapping projecting part for projecting the secondary charged particle and/or the reflected charge particle in some degree of magnification on the electron image detecting part; and an inspection part for inspecting the substrate on the basis of the picture signal, the electron image detecting part including a charged particle multiplying device for multiplying the secondary charged particle and/or the reflected charged particle, and an image grabbing element having a fluorescent body for receiving the multiplied secondary charged particle and/or reflected charged particle as the electron image and for converting the electron image into an optical image, the image grabbing element converting the optical image into the picture signal, the charged particle multiplying device having an entrance surface through which the secondary charged particle and/or the reflected charged particle enter, the fluorescent body having a light receiving surface for receiving the multiplied secondary charged particle and/or reflected charged particle and a fluorescent surface on which the optical image appears, the method comprising: causing the fluorescent surface of the fluorescent body to be grounded, and applying a first negative potential to the entrance surface of the charged particle multiplying device.